Final answer:
The annulus fibrosus is the collagen-rich structure that limits the expansion of the nucleus pulposus during spine compression.
Step-by-step explanation:
The collagen-rich structure that limits the expansion of the nucleus pulposus when the spine is compressed is the annulus fibrosus. The annulus fibrosus is the tough, fibrous outer layer of an intervertebral disc, and it is firmly anchored to the bodies of the adjacent vertebrae. The nucleus pulposus, on the other hand, is a softer, more gel-like material that resides within the annulus fibrosus and has a high water content, which helps it resist compression and bear weight. As people age, the water content of the nucleus pulposus declines, causing the disc to thin, which can affect flexibility and range of motion.